The present invention relates to a color transmission system discrimination circuit in a television receiver designed for a plurality of television systems.
In general, a color system or a color transmission system refers to three systems such as PAL, SECAM, and NTSC. Actually, including variations of frequencies of a reference sub-carrier signal, about 10 kinds of color transmission systems have been used in the world. A reference sub-carrier signal has 4 kinds of frequencies: 3.58 MHz in the NTSC system, 4.43 MHz in the PAL system, a frequency slightly lower than 3.58 MHz in the PAL-M system, and a frequency slightly higher than 3.58 MHz in the PAL-N system.
Recently, television receivers designed for a multi color-transmission system have been developed and put into practical use, which are capable of carrying out processing in accordance with the above-mentioned plurality of kinds of color transmission systems. Such a receiver typically is equipped with a color transmission system discrimination circuit for determining the kind of a color transmission system that is being received and automatically switching the setting of a signal processing circuit.
Japanese Laid-Open Publication No. 6-351024 describes a conventional color transmission system discrimination circuit of this kind. In this prior art, it is automatically determined which color transmission system (PAL, SECAM, NTSC, or 4.43 MHz-modulated NTSC) is being received, based on a software algorithm of a microcomputer.
According to the above-mentioned discrimination algorithm, the settings of a plurality of color transmission systems are switched in a predetermined order. After the respective settings are switched, the presence/absence of a color killer signal is detected based on information of a color killer voltage. As a result, if it is found that the set system is correct, its setting is maintained. If the setting is not correct, it is switched to the subsequent setting.
However, the above-mentioned conventional discrimination circuit has several problems. First, a color killer voltage is checked when a predetermined period of time has passed after switching the setting of a color transmission system. This processing is repeated on a color transmission system basis. Therefore, it takes a long period of time for a color transmission system to be correctly set.
Second, only one circuit is provided for detecting a color killer voltage, which makes it necessary to conduct switching between the detection of a burst signal (color signal) in a 180xc2x0 phase in the NTSC system and the detection of a burst signal in a 90xc2x0 phase in the PAL system. This lengthens the period of time for a color transmission system to be correctly set, and leads to a detection error in the SECAM system.
The third problem is related to the second one. That is, the third problem is difficulty in detecting more color transmission systems. For example, the SECAM system is determined by using the detection result of whether a vertical video frequency is 50 Hz or 60 Hz. However, according to this method, the SECAM system of 60 Hz cannot be determined. Actually, no countries have adopted this system as far as a ground wave is concerned. Some broadcasting stations are able to adopt a combination of their own color transmission systems, along with the widespread use of CATVs.
Under the above-mentioned circumstance, it has been desired to exactly and rapidly determine which color transmission system is being received among possible combinations of three systems: PAL, SECAM, and NTSC (four systems including monochrome broadcasting) and a frequency of a reference sub-carrier signal. The objective of the present invention is to provide such a color transmission system discrimination circuit.
In the following description, for convenience, a xe2x80x9ccolor systemxe2x80x9d refers to three systems: PAL, SECAM, and NTSC (four systems including monochrome broadcasting). In a broad sense including variations of frequencies of a reference sub-carrier signal, a xe2x80x9ccolor systemxe2x80x9d refers to a xe2x80x9ccolor transmission systemxe2x80x9d.
A color transmission system discrimination circuit of the present invention in a television receiver capable of receiving a plurality of television systems, includes: a 180xc2x0 killer signal detection portion for detecting the presence/absence of a burst signal in a 180xc2x0 phase and outputting a first color killer signal; a 90xc2x0 killer signal detection portion for detecting the presence/absence of a burst signal in a 90xc2x0 phase and outputting a second color killer signal; a SECAM killer signal detection portion for detecting the presence/absence of a SECAM signal corresponding to a SECAM system and outputting a third color killer signal; and a discrimination processing portion for determining a color system that is being received and a frequency of a reference sub-carrier signal, using the first to third color killer signals. Because of this structure, compared with a conventional discrimination method in which the PAL system and the NTSC system use a color killer signal in common, the discrimination precision is improved, and the time required for discrimination is shortened.
In a more specific structure, it is preferable that the discrimination processing portion sets a provisional color transmission system in a first stage and captures a required signal among the first, second, and third color killer signals thus obtained, sets a provisional color transmission system in the second stage, which is different from the provisional color transmission system in the first stage, and captures a required signal among the first, second, and third color killer signals thus obtained, sets a provisional color transmission system in the third stage, which is different from the provisional color transmission systems in the first and second stages, and captures a required signal among the first, second, and third color killer signals thus obtained, and determines a color system which is being received and a frequency of a reference sub-carrier signal, based on a combination of the color killer signals captured in each of the stages.
In a more specific preferable circuit configuration, the discrimination processing portion includes: a killer state change detection circuit for detecting a change in a color killer signal; a sequencer which is driven with an output signal of the killer state change detection circuit and performs the first, second, and third stages in this order respectively for a predetermined period of time; a first latch circuit for latching the color killer signal captured in the first stage; a second latch circuit for latching the color killer signal captured in the second stage; and a discrimination decoder for outputting a discrimination signal of a color system and a discrimination signal of a frequency of a reference sub-carrier signal, based on the color killer signal captured in the third stage and an output signal of the first and second latch circuits.
More preferably, the above-mentioned transmission system discrimination circuit further includes: a third latch circuit for latching a discrimination signal output from the discrimination decoder, a mode setting circuit for provisionally setting a color transmission system in each of the stages; and a switch for selecting an output of the latch circuit or an output of the mode circuit and switching the selected output to an output signal for setting a color transmission system. The switch selects an output of the mode circuit in the first to third stages, and the switch selects an output of the latch circuit in a fourth stage following the third stage. In this case, a color transmission system is set by switching based on a discrimination signal from the discrimination decoder in the fourth stage.
In the case where a discrimination signal output from the discrimination decoder represents monochrome broadcasting, it is preferable that the sequencer returns to the first stage without performing the fourth stage. According to this structure, when monochrome broadcasting is switched to color broadcasting, a discrimination result already has been output from the discrimination decoder. Therefore, a color transmission system is determined in a short period of time, and a predetermined switching of settings is conducted.
Furthermore, it is preferable that the above-mentioned color transmission system discrimination circuit includes means for, in the first to third stages, fixing a trap frequency of a trap filter circuit for removing a color signal component from a video composite signal so as to prevent disturbance of the screen and the appearance of color noise, fixing a color demodulation circuit to a PAL/NTSC circuit or a SECAM circuit, and setting a color level of the color demodulation circuit and a color amplifier at a minimum value.
Furthermore, in each of the above-mentioned structures, it is preferable that a color transmission system is provisionally set in the second stage so as to have higher priority (e.g., possibility), compared with the first stage, and a color transmission system is provisionally set in the third stage so as to have higher priority, compared with the second stage. Because of this, in the case where a color transmission system is switched at random timing, as the priority of the switched color transmission system is higher, the period of time required for discrimination and decision becomes shorter in terms of probability.
A part of the above-mentioned structure can be performed by a software program.